A difficult obstacle associated with the exploration and production of oil and gas is management of significant ocean currents. These currents can produce vortex-induced vibration (VIV) and/or large deflections of tubulars associated with drilling and production. VIV can cause substantial fatigue damage to the tubular or cause suspension of drilling due to increased deflections. While helical strakes, if properly designed, can reduce the VIV fatigue damage rate of a tubular in an ocean current, they typically produce an increase in the drag on the tubular and hence an increase in deflection. Thus, helical strakes can be effective for solving the vibration problem at the expense of worsening the drag and deflection problem.
Another solution is to use fairings as the VIV suppression device. A properly designed fairing can reduce both the VIV and the drag. Fairings are usually made to be free to weathervane around the tubular with changes in the ocean current.
Virtually all fairing designs and many helical strake designs for use on vertical or inclined tubulars require a collar. The collar is sufficiently tightened around the tubular so that sliding along the tubular due to gravity, environmental forces, or impact from adjacent appurtenances, is prevented. Collars may be made in a variety of ways, but their primary function is to grip the tubular with sufficient force to keep VIV suppression devices, or other appurtenances, from sliding past the collar.
A difficulty with collar design is keeping sufficient force on the tubular when the tubular outside diameter changes due to hydrostatic pressure. As a tubular is lowered into the ocean, it experiences a compressive force that can reduce the diameter by compressing: the tubular; a coating on the tubular; insulation on the tubular; buoyancy on the tubular; or other material that is attached to the tubular. If the collar is not designed to accommodate changes in the tubular diameter due to hydrostatic pressure, then the collar can lose its ability to sufficiently grip the tubular. Some tubulars, such as drilling risers, may be lowered into the water and later raised from the water, and thus the hydrostatic pressure can change.
Another difficulty with collar design is providing sufficient gripping force along the length of a tubular where the diameter changes in small amounts along the tubular. These variations may be due to: changes imposed by design; variations in the diameter during manufacturing; changes in diameter due to repairs made to the tubular or its coatings, insulation, or buoyancy; changes in material; changes in material stiffness, or anyone of a number of reasons that can cause the diameter to vary.